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Q: How long will carbon dioxide stored in geological formations remain sequestered?

A: Permanently.

In nature, we find carbon dioxide that’s been stored in geological formations for tens of millions of years. Some examples of places where carbon dioxide has accumulated and remained stored underground are the Jackson Dome in Mississippi, the Bravo Dome in Oklahoma, and the McElmo Dome in Colorado.

Carbon dioxide is trapped in these geological formations the same way that oil and natural gas are trapped. In most places deep underground, the rock is saturated with salty water called brine. Like oil and natural gas, carbon dioxide is less dense than the brine. It is buoyant and floats on the brine. So CO2 naturally bubbles upward wherever it can, just like natural gas.

What stops it? Sedimentary basins where CO2 would be stored are layered. Some layers allow fluid to soak into them and move through them; they are called permeable. Between the permeable layers are rock layers that are impermeable to all fluids except water; they do not let CO2, oil or natural gas move through them.

Buoyant fluids produced deep below the earth’s crust rise upward through rock containing brine until they encounter a layer of rock that is too impermeable for them to pass through. And, like a helium balloon bumping against the ceiling at a kid’s birthday party, buoyant fluids like CO2 remain trapped there.

It might not be the most dignified analogy, but a baby diaper is a similar system. Some of the layers soak up the fluids. Others prevent them from getting out.

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Carbon dioxide, and other buoyant fluids like oil and gas often remain stored (or sequestered) below impermeable rock that prevents them from moving upward, called caprock.

Just how impermeable is this caprock? Scientists working in the North Sea studied a carbon dioxide reservoir that accumulated 70 to 80 million years ago 4 km (2.4 miles) below the surface. They used stable isotopes to trace the movement of the carbon dioxide through a type of caprock called mudrock.

The study found that carbon dioxide permeates the mudrock layer above it at a rate of about 1 millionth of a gram per cubic centimeter in a year. This translates to an upward movement of 12 meters or about 40 feet in 70 to 80 million years.

Computer models show similar results. In 2010, scientists in Germany evaluated effects of thickness and porosity of caprock on CO2 movement. They found that even in the case of the thinnest (10 m) and most porous (effective diffusion coefficient 10-10 m2/s) realistic caprock, carbon dioxide would remain sequestered for 100,000 years. They concluded that “diffusive losses through even this caprock layers are negligible for CO2 storage operations.”

But, the earth’s crust is not a static box. It shifts. Faults form. Fluids move.

When these processes occur, there is a possibility that buoyant fluids can leak. We know this because we find carbon dioxide, as well as oil and natural gas, leaking from depth to the earth’s surface in some places. Operators who sequester carbon dioxide are required to locate storage sites in places that will not leak. Permits required by regulators, like the US EPA Underground Injection Control Program, provide evidence that there are no pathways to the surface before an injection project even begins.

What if the permitting process failed, and CO2 was stored in a site that was later found to be leaking? We can understand this case by analogy places where pathways to the surface have formed and buoyant fluids naturally leak from geologic formations. [link to FAQ on natural analogs] Dr. Steve Bryant, professor of petroleum engineering at The University of Texas at Austin, explains, “Stored carbon dioxide will not erupt. It will seep slowly to the surface.”

One place where such slow leakage occurs is in the Sacramento Valley of California. The Rio Vista Gas Field contains roughly 100 naturally formed gas seeps, although today only 40 are active. The subsurface reservoir holds 100 billion cubic meters of methane gas. Methane is more buoyant than carbon dioxide, and it also dissolves less readily in water. Although methane behaves slightly differently than carbon dioxide, it helps put boundaries on rates of leakage. Scientists measured the flow through the seeps at 30 to 85 cubic meters per day. It would take 100,000 years for all the methane in the reservoir to trickle to the surface.

That’s the kind of time scale that makes sense in the effort to mitigate climate change. Dr. Eric Lindeberg at SINTEF Petroleum Research, an independent research foundation in Norway, ran climate change models varying the amount of time carbon dioxide would remain sequestered. The study showed that if all the existing reservoirs of fossil fuel are burned, then retaining CO2 underground for at least 7,200 years will minimize the effects of climate change. Since the time scale of geological sequestration ranges from tens of thousands of years to indefinitely, it much exceeds the time required to make a difference.

If we burn all the fossil fuels on the planet, and then stored the CO2 for at least 7,200 years, we could keep temperature increases below 3 degrees Celsius (black line). Models that include transition to alternative energy sources (not shown) decrease the required retention time.